<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        div {
            width: 200px;
            height: 200px;
            background-color: #f5f5f5;
            border: 20px solid #ccc;
            /* 使盒子的大小为width+padding+border */
            box-sizing: content-box;
        }
        p {
            width: 200px;
            height: 200px;
            background-color: #f5f5f5;
            border: 20px solid #ccc;
            /* 使盒子的大小为width padding和border无法拓宽盒子 */
            box-sizing: border-box;
        }
    </style>
</head>
<body>
    <div></div>
    <p></p>
</body>
</html>